Stray bodies in a nuclear reactor may, if no corrective action is taken, cause considerable damage by hammering on the structures.
A prior art method for detecting stray bodies consists in monitoring and measuring, at regular intervals (for example every day), signals supplied by sensors (generally accelerometers) placed in contact with a wall of the primary circuit of the reactor.
That approach has limitations. Except if the frequency of the monitoring operations is increased, the presence of a stray body may be detected only with a delay such that considerable damage has already occured. And it is difficult to attribute abnormal noises to a specific origin among those which are possible and which comprise vapor leaks and shocks due to non dangerous disturbing phenomena.
European Specification No. 00 49659 discloses an apparatus having several sensors each supported by a separate part of the reactor and each associated with a detection channel comprising a discriminator. All channels are connected to encoding means which classify the pulses from the different sensors in the order of arrival and compare the sequences obtained with a predetermined sequence. However, the shocks due to stray bodies cannot be identified when there is a high level of background noise.
French Specification No. 2 215 264 discloses a detection apparatus which determines the rate of the impacts detected by a sensor and their energy and which only measures the energy extremum for a given time. With this approach, discrimination as to the origin of the impacts is not possible.
An apparatus currently used by the assignee of this application detects abnormal vibrations and transients in a reactor vessel by measuring the peak factor of the vibrations applied to an accelerometer in contact with a wall of the primary circuit of the reactor. An alarm is triggered off when this factor exceeds a given value. However, the impacts cannot be differentiated as to their origin and, in particular, it is just as sensitive to the impacts due to oscillation of mechanical parts which are permanently secured as to the presence of a stray body.
It is an object of the invention to provide an improved process and installation. It is a more specific object to provide for continuous discrimination between the normal noise sources and the sources of impact noises due to stray bodies.
For that purpose, the invention uses the fact that the impacts detected by sensors, typically accelerometers, supported by parts immersed in the reactor but uncoupled from the external wall of the primary circuit, supply, in response to impacts by stray bodies against the wall of the primary circuit, correlated signals since they energize the different sensors at times which only differ by the travel time differences of the disturbance.
On the other hand, the impacts due to oscillations or vibrations of the structure carrying a detector do not give rise to correlated signals on all the detectors.
The invention uses this fact in combination with another: in the absence of impact, the vibratory background noise is random in nature and the ratio between the peak value and the effective or rms value, which is called "peak factor", does not exceed 4 to 5 because of the statistical properties of the signal. Consequently, the impact may be differentiated from the continuous background noise, even loud, because they have a peak factor higher than a threshold. Since the background noise is close to four times the effective or rms value, a peak factor threshold may be chosen equal to 12, so as to obtain good discrimination of the impacts.
According to the invention, there is provided a process for detecting stray bodies in a nuclear reactor wherein signals supplied by at least two mechanical impact sensors, such as accelerometers, are directed to individual channels. Those signal pulses which have a peak factor higher than a predetermined level are detected and the coincidence rate is determined between said signals in the two channels so as to cause an alarm when said rate exceeds a threshold.
The threshold for the coincidence rate will be chosen so as to avoid untimely alarms due to accidental coincidences. Similarly, a peak factor level (ratio between the peak value of the signal and the effective value) will be chosen so as to separate out the pulse signals due to the impacts of the background noise which is variable depending on the operation state of the reactor and which may be high in the case of vapor leaks in the primary or secondary circuit.
In general, at least in the case of a reactor using water as moderator and as cooling agent, use will be made of the coincidences between detectors supported by guide tubes passing through the wall of the vessel of the reactor. Thus the impacts due to a stray body, may be differentiated from the impacts caused by an internal instrumentation glove finger of the reactor striking against the foot of the corresponding fuel assembly. Such an impact is transmitted to the bottom of the vessel through the guide tube, but is not felt by the sensors supported by other guide tubes.
To sum up, the invention associates the determination of the peak factor, a parameter which is justified physically, from the instantaneous measurement and determination of the effective value determined from the prior measurement, and correlation between several channels. The time constant of the circuit for emasuring the effective value (rms detection circuit) will be chosen so that the presence in the signal of spaced pulses does not in practice modify this value.
The process may also advantageously comprise comparison of the effective value of the signal with a second threshold. The process thus allows even very closely spaced impacts to be detected (caused for example by the presence of several stray bodies). In this case, in fact, the effective value increases and the peak factor decreases. But the increase in the effective value causes this latter to exceed a detection threshold, which will for example be equal to three times the normal effective value, in the absence of stray bodies, and triggers off an alarm.
The invention also provides an installation for detecting stray bodies in the vessel of a nuclear reactor, for implementing the above defined process, comprising several mechanical pulse sensors, such as accelerometers, supported by separate structures of the vessel, which installation is characterized in that it comprises means for detecting coincidences between pulse signals supplied by the different sensors and having a peak factor greater than a predetermined level and means for causing an alarm in response to overshooting of a predetermined threshold by the coincidence rate.
The invention will be better understood from reading the following description of a particular embodiment, given by way of non limitative example.